In humans, computed tomography (CT) is a known diagnostic tool for patients with clinical signs of thoracic disease. Compared with thoracic radiography, CT images have superior contrast resolution and anatomic superposition is not a problem. Pathologic changes that are not apparent on survey radiographs may be detected using CT. CT is also used to differentiate differences between pleural, extrapleural, or mediastinal tissues from that of lung tissue.
When choosing protocols for CT in humans there is always a trade-off between image quality and dosage. Radiologists and technicians must balance protocol selection between resolution, noise, and contrast to achieve good image quality and keep patient exposure as low as reasonably achievable. In pediatric CT there is a recommendation for using a sharper algorithm for reconstruction of lung images. Sharper algorithms delineate object margins more clearly at the expense of increased image noise. Less sharp algorithms reduce noise, allowing larger low contrast object to become more visible although edges will be more blurry and fine detail lost. In veterinary medicine radiation exposure is not as much of a concern as it is in humans, however, studies evaluating radiation dose and safety associated with routine imaging diagnosis are lacking.
In veterinary medicine, survey thoracic radiography is the standard imaging modality used for evaluating the thorax, even though the use of CT to diagnose thoracic diseases in anesthetized cats is known. This is due, at least in part, to the need for general anesthesia when performing CT, because movement by a non-sedated animal patient commonly causes significant artifacts, which usually appear as shading or streaking in the reconstructed image. However, general anesthesia has inherent risks and is sometimes contraindicated in emergency conditions, especially for patients in respiratory distress. General anesthesia often causes varying degrees of atelectasis that can mimic or obscure underlying disease. Additional CT scans in different positions may be necessary to evaluate the patient fully, thereby increasing imaging and anesthesia time.